1. Field of the Invention
This invention relates generally to information recording and reproducing apparatus and, more particularly, is directed to a circuit for separating simultaneously reproduced signals having at least partially overlapping frequency bands.
2. Description of the Prior Art
It is known to employ an 8-mm video tape recorder (VTR) for the reproducing of digital data recorded on a magnetic tape. In such 8-mm VTR, an FM audio signal is interposed between the frequency band of an FM luminance signal and the frequency band of a low-frequency converted chrominance signal, and the resulting combined signal, which is hereinafter simply referred to as the video signal, is recorded in the main portions of successive adjacent, oblique or slant tracks extending across a magnetic recording tape. It is further known that, in order to record and/or reproduce audio signals of higher quality by means of the 8-mm VTR, the audio signal is PCM-encoded, time-base compressed and recorded in overscan sections situated at one end of the oblique record tracks, for example, as disclosed in U.S. Pat. No. 4,551,771, issued Nov. 5, 1985, and having a common assignee herewith. However, in the known arrangement in which the audio signal is PCM encoded for recording in the existing 8-mm VTR, the sampling frequency is twice the horizontal frequency of the video signal, that is, approximately 31.5 kHz in the case of a standard NTSC video signal, and the PCM audio signal is quantized in non-linear eight bits.
On the other hand, in other digital audio recording and/or reproducing apparatus, such as, digital audio tape (DAT) recorders and compact disk (CD) players the adopted standards for the PCM audio signal provide a sampling frequency of 44.1 kHz or 48 kHz and a quantized bit number of 16 bits, for example, as shown in U.S. Pat. No. 4,577,180 issued Mar. 18, 1986, or U.S. Pat. No. 4,617,552, issued Oct. 14, 1986, and each also having a common assignee herewith. Therefore, the quality of the PCM audio signal, as recorded and reproduced by the known 8-mm VTR, is not as high as the quality of the audio signals obtained with DAT recorders or CD players.
Although it has been proposed to convert the 8-mm VTR audio signal to a PCM signal having a standard similar to that employed in the other digital audio apparatus, such as, CD players and DAT recorders, the quantity of data that then needs to be recorded is increased to more than twice that characterizing the actual PCM signal. However, it is not possible to modify either the magnetic characteristic of the tape or the length of the overscan section of each track in which the PCM audio signal is recorded so that a different method needs to be employed for recording the increased amount of data representing the high-quality PCM audio signal. More particularly, in the case of the standard PCM audio signal recorded in the overscan section of each record track by means of the 8-mm VTR, such standard PCM audio signal is converted, for recording, to a bi-phase mark signal which, for example, as the frequency band indicated in broken lines at Sb in FIG. 1B. On the other hand, when a high-quality PCM audio signal is to be recorded by means of an 8-mm VTR, such signal is recorded, for example, in the form of an NRZ signal. Since the NRZ signal occupies a relatively narrower band width, even when the quantity of data is increased, as for the high-quality PCM audio signal, the corresponding NRZ signal will merely occupy a frequency band, for example, as indicated by the solid line Sn on FIG. 1B. Therefore, the NRZ signal corresponding to the high-quality PCM audio signal can be recorded and reproduced by the 8-mm VTR in place of the originally employed PCM signal, that is, the PCM signal having a sampling frequency of 31.5 kHz and which is quantized in non-linear eight bits.
However, the NRZ signal Sn includes a D.C. component and the increase in the quantity of data for recording and reproducing a high-quality audio signal causes the lower band components of the NRZ signal to be correspondingly increased. The lower band region of the NRZ signal includes a tracking pilot signal Sp used for automatic track following (ATF) during reproducing or playback and having a frequency which is approximately in the range between 103 kHz and 165 kHz, for example, as disclosed in U.S. Pat. No. 4,509,083 and U.S. Pat. No. 4,658,309. Therefore, if the pilot signal Sp is added to the NRZ signal Sn for recording simultaneously with the latter in the overscan section of each track, and if the reproduced pilot signal is separated from the reproduced NRZ signal on the basis of the frequency thereof, for example, by a band pass filter, the separated reproduced pilot signal inevitably includes lower band components of the reproduced NRZ signal which results in tracking errors. The reproduced NRZ signal Sn also includes the pilot signal Sp, for example, as shown in FIG. 2A, which increases the error rate of the PCM audio signal.
In order to avoid the foregoing problems, in the DAT recorder in which the frequency bands occupied by the PCM audio signal and the pilot signal at least partially overlap, the pilot signal is recorded separately from the PCM signal, for example, in front and rear portions of the recording area for the PCM signal. However, such arrangement for ensuring that the reproduced PCM signal and pilot signal can be satisfactorily separated or time-demultiplexed reduces the area of the recording medium available for receiving the PCM signal and thereby reduces the recording density of the PCM signal.